In recent years, power source systems in which a large number of secondary batteries are connected in series to output a high voltage have been widely used. The power source system is typified by vehicular secondary batteries installed as power sources on hybrid cars using an engine together with an electric motor or electric automobiles. Such a power source system is configured by connecting a large number, for example, 80 cells to 500 cells, of secondary batteries in series. Therefore, the reliability of the entire battery system is difficult to ensure.
More specifically, in such a power source system, there is variation in the stored charge amount which is charged in the secondary batteries. The variation is due to variation in characteristics which occurs in a manufacturing process of secondary batteries constituting the power source system.
When the power source system is repeatedly charged and discharged in such a state where there is variation in the stored charge amount, the degradation of the secondary battery whose stored charge amount is small may be accelerated. More specifically, when charging the power source system, a secondary battery, which has greater charge amount than another secondary battery prior to charge, will be fully charged earlier than the other secondary battery. Therefore, this secondary battery is easily to be subjected to overcharge. Thus, there is a risk of accelerating degradation of a secondary battery with a greater stored charge amount. On the other hand, when discharging the power source system, a stored charge amount of a secondary battery, which has smaller charge amount than another secondary battery prior to discharge, will become zero earlier than the other secondary battery. Hence, this secondary battery is easily to be subjected to over-discharge. Thus, there is a risk of accelerating degradation of a secondary battery with a small stored charge amount.
In the secondary battery with accelerated degradation, capacity is reduced and service life is shortened. When some of the secondary batteries of the power source system are degraded, the entire power source system cannot be used and reliability is reduced. Therefore, the effect produced by the degradation of secondary batteries caused by the variation in the stored charge amount is large. For this reason, in a power source system using a large number of secondary batteries, it is desirable to equalize the stored charge amount among the secondary batteries, that is, equalize the terminal voltage among the secondary batteries.
A technique for equalizing the terminal voltage among secondary batteries is known, as described hereinbelow (see, for example, Patent Document 1).
With the technique described in Patent Document 1, the terminal voltage of the entire battery pack configured by connecting cells in series is divided by voltage-dividing resistors. The number of the resistors corresponds to the number of cells. As a result, the divided voltage becomes a voltage obtained by averaging the terminal voltages of the cells. The average voltage thus obtained is compared with the terminal voltage of each cell. The terminal voltages of the secondary batteries are equalized by discharging the cell with a terminal voltage higher than the average voltage.
However, the inconvenience associated with the technique described in Patent Document 1 is that when the number of cells used in a battery pack is changed, the number of voltage-dividing resistors or the resistance value thereof should be changed, which makes it difficult to change the number of cells.
Patent Document 1: Japanese Patent Application Publication No. 2001-95169